Electrolytic process for hard surfacing aluminum



United States Patent ELECTROLYTIC PROCESS FOR HARD SURFACIN G ALUMINUMPaul Hunt Taylor, Gardena, Calif.

No Drawing. Application July 7, 1958 Serial No. 746,664

15 Claims. (Cl. 204- 58) This invention relates to the treatment ofaluminum surfaces to produce thereon oxide coatings.

Methods have been heretofore proposed and used for the treatment ofaluminum surfaces to produce so-called oxide coatings thereon. However,in most cases, the coatings produced by the usual methods ofelectrolytic oxidation are substantially porous and lack the hardness"desirable for many applications, and the usual methods are limited intheir applicability to aluminum base alloys having low percentages ofalloying elements. One of the of the surface, such as by electrolyticoxidation.

Another object of this invention is to provide an electrolytic processfor the production of highly abrasion and corrosion resistant oxidecoatings on the surfaces of virtually all types of aluminum base alloys.

Other objects and advantages of this invention it is 1 believed will bereadily apparent to those skilled in the art from the following detaileddescription of a preferred embodiment thereof.

Briefly this invention comprises the discovery that the addition ofrnellitic acid to the conventional sulfuric acid solution electrolyteused in the electrolytic oxidation unexpectedly results in theproduction of hard, dense oxide coatingson aluminum surfaces, even inthe case of aluminum base alloys containing amounts of alloying elementsheretofore considered to be unsatisfactorily high for propor treatment.It has also been discovered that the addi- ;tion of mellitic acid toaqueous solutions of chromic,

phosphoric and oxalic acids used as the electrolyte in 7 theelectrolytic oxidation unexpectedly results in the production of hard,dense oxide coatings on aluminum surfaces. The electrolyte to which thernellitic acid is added ma also comprise mixtures of one or more of thesulfuric, chromic, phosphoric and oxalic acids in all propor- ,tions.Additionally, proper concentrations of rnellitic acid and theseelectrolyte acids have been discovered.

In carrying out the process of this invention, conventional electrolyticoxidation techniques are employed. That is, the aluminum part to betreated is made the anode where direct current is used, or the aluminumparts .may be used as both electrodes in the event that alternatingcurrent is employed.

In carrying out the process of this invention, the aluminum parts to betreated are completely immersed in the electrolyte which is contained ina tank of stainless steel or lead-lined cold rolled steel, the tankserving as the cathode. The aluminum parts are supported from .andmaintained in electrical contact with the convenjtional copper bus barsby means of clips or brackets of an ice aluminum alloy, preferably onewhich polarizes at a higher voltage than the alloy being treated. It ispre ferred to maintain the temperature of the electrolyte between about5 F. and about 45 F. The electrolyte itself comprises an aqueoussolution of rnellitic acid and sulfuric acid. For best results it hasbeen found that the concentrations should be maintained between about0.5 and about 1.5% mellitic acid by weight, and between about 7.0 andabout 13.0% sulfuric acid by weight. The preferred concentration ranges(by weight) of the other electrolyte acids are: chromic acid, 7.0-10%;phosphoric acid, 1040%; oxalic acid, 50-10%. As will be known to thoseskilled in the art, rnellitic acid may be prepared from charcoal orgraphite by oxidation with potassium permanganate or nitric acid.

The electrolyte is vigorously agitated to prevent local over-heatingduring electrolysis. It is preferred to slowly polarize the aluminumparts by initially applying to the cell the lowest possible currentdensity consistent with flow of current in the bath. This currentdensity is maintained for a minimum time of about 5 minutes whereuponthe current is increased at the rate of approximately 6.0 amperes persquare foot (area of parts plus area of immersed clamp portions) everythree minutes until the desired current density is reached. With thesulfuric acid electrolyte and where hard coatings are desired a currentdensity of 36.0 amperes per square foot is sufficient. A current densityof at least 40 amperes per square foot should be used with theotherelectrolyte acids. These densities are maintained for varying periods oftime, depending upon the thickness of coating desired. For example, a20-minute period will produce a coating of 0.0012 inch in thickness,while a 40-minute treatment will produce a coating 0.0025 inch thick. Ifthinner coatings (i.e., less than about 0.001 inch) are required,

lower current densities may be employed.

It Will be understood that during electrolysis, the electrolyte isdepleted, and accordingly, additions of the mellitic and sulfuric and/orthe other acids must be made to maintain the concentration limits.

Example 1 As a specific example of the process of this invention,aluminum parts (2024 aluminum alloy) to be treated were degreased withtrichloroethylene, cleaned in a buffered soap cleaner in water solutionat F., and thoroughly rinsed in clean, hot tap Water. The parts were notdried, but immediately attached to the bus bars by means of the clipsand fully immersed in the electrolyte which consisted of an aqueoussolution of 1.0% rnellitic acid by weight and 10% sulfuric acid byweight. The temperature of the bath was maintained at 25 F. throughoutthe process. Agitation thereof was begun and current applied at adensity of 6.0 amperes per square foot for 5 minutes, the current thenbeing increased in increments of 6 amperes per square foot each threeminutes until the total of 36.0 amperes per square foot was reached,electrolysis being continued at this value for about 20 minutes. Thecurrent was then discontinued, the parts were removed and thoroughlyrinsed in cold tap Water, drained and dried at a low temperature.

Example 2 The process of this example was identical to that of Example1, except that 7% by weight chromic acid was substituted for thesulfuric acid, the initial current density was 3 amperes per square footand the final current den sity was 40 amperes per square foot.

Example 3 The process of this example was identical to that of Example1, except that 15% by weight phosphoric acid 3,. was substituted for thesulfuric acid, and bath temperature was 10 F., the initial currentdensity was 10 amperes per square foot and the final current density was40 amperes per square foot.

Example 4 The processof this example was identical to that of Example 1,except that 6% byweight of oxalic acid was substituted for the sulfuricacid, 0.5% by weight of mellitic acid was used, the initial currentdensity was 10 amperes per square foot and the final density was 40amperes per square foot.

The addition of mellitic acid to the solution of sulfuric acidmaintained as specified herein makes it possible to place a protectivesurface on virtually all aluminum base alloys as well as pure aluminum.Other processes of this typeare limited in the applicability due to highpercentages of alloying elements. This process does not have theselimitations. The coating can be applied to aluminum base alloys such asthe following which cannot be satisfactorily treated by the processheretofore used:

(1) Those whose copper content is 3.0 percent or more.

(2) Those whose silicon content is over 7.0 percent.

(3) Those whose zinc content is as much as .0 percent.

(4) Those whose copper content is 4.5 percent and nickel content is 2.0percent.

(5) Those whose magnesium content is as much as 7.0 percent, and

(6) Any aluminum alloy whose percentage of alloying elements does notexceed 16.0 percent.

It is believed thatmellitic acid is reduced by electrolysis during theoperation of the electrolyte. It is reasoned thatpyromellitic andmellophanic acids may be formed due to this reduction and that one, two,or all "three acids produce an even distribution of current which allowsthe coating of the high element bearing alloys.

The addition of the mellitic acid increases the conductivity of the bathand increases the throwing power of the electrolyte, thus greatlyfacilitating the coating of parts of intricate shapes.

The oxide coatings produced by the process of this invention have thefollowing properties:

(1) Coatings of .001 inch or more will withstand corrosion of the basemetal 6000 hours or more in a vapor of 20 percent sodium chloride indistilled water at 95 degrees F., pH 6.5 to 7.2. The coatings aretherefore, highly corrosive resistant.

(2) When given an abrasion test using the Taber abraser and wheels CS#17and 1000 grams weight and 70 r.p.m., a coating of .001 inch willwithstand 15,000 or more revolutions before penetration to the basemetal.

(3) The coatings will absorb a variety of dry lubricants such asmolybdenum disulfide dispersed in phenolic resln.

(4) The mechanical properties of the alloy are decreased by coatings.001 inch or more in thickness. The fatigue strength, percent ofelongation, and tensile strength are reduced.

(5) The coatings will accept water soluble dyes, but colors are limiteddue to the dark shades of the coatings.

(6) The hardness of a .002 inch coating will lie between 29 and 55Rockwell C when measuredwith a Tukon hardness tester and converted.

(7) The coating is highly dielectric and a coating of .002 inch willwithstand in excess of 1000 volts before penetration to the base metal.

(8) Definite dimensional growth is associated withthe production of thecoating and is especially evident'with coatingsof .001 inch or more inthickness. This dimensional growth is roughly 50 percent of the totalcoating thickness and varies somewhat with the alloy as well as theoperating conditions of the electrolyte; however, variations are slight.

(9 The coatings are excellent paint bases and will accept lacquer orenamel without the use of a primer coating.

This application is a continuation-in-part of my copending applicationSerial Number 427,352 filed May 3, 1954, on Electrolytic Process forHard Surfacing Aluminum, now abandoned which in turn is a continuingapplication, based on my co-pending application Serial Number 357,630,filed May 26, 1953, on Hard Surfacing Process for Aluminum, nowabandoned.

Having fully described my invention, it is to be understood that I donot wish to be limited to the details set forth, but my invention is ofthe full scope of the appended claims.

I claim:

1. In a process for the production of an oxide coating on an aluminumsurface, the step comprising subjecting said surface to electrolyticoxidation in an electrolyte consisting essentially of an aqueoussolution of mellitic acid and an acid selected from the group consistingof sulfuric acid, chromic acid, phosphoric acid, oxalic acid andmixtures thereof.

2. In a process for the production of an oxide coating on an aluminumsurface, the step comprising subjecting said surface to electrolyticoxidation in an electrolyte consisting essentially of an aqueoussolution of between about 0.5 and about 1.5% by weight of mellitic acidand between about 7.0 and about 13.0% by weight of sulfuric acid.

3. In a process for the production of an oxide coating on an aluminumsurface, the step comprising subjecting said surface to electrolyticoxidation in an electrolyte consisting essentially of an aqueoussolution of between about 0.5 andabout 1.5% by weight of melliticacidand between about 7.0 and about 10% by weight of chromic acid.

4. In a process for the production of an oxide coating on an aluminumsurface, the step comprising subjecting said surface to electrolyticoxidation in an electrolyte consisting essentially of an aqueoussolutionof between about 0.5 and about 1.5% by weight of mellitic acidand between about 10 and about 40% by weight ofphosphoric acid.

5. In a process for the production of an oxide coating on an aluminumsurface, the step comprising subjecting said surface to electrolyticoxidation in an electrolyte consisting essentially of an aqueoussolution of between about 0.5 and about 1.5% by weight of mellitic acidand between about 5.0 and about 10% by weight of oxalic acid.

6. In a process for the production of an oxide coating on an aluminumsurface, the step comprising subjecting said surface to electrolyticoxidation in an electrolyte consisting essentially of an aqueoussolution of mellitic acid and an acid selected from the group consistingof sulfuric acid, chromic acid, phosphoric acid, oxalic acid andmixtures thereof, said electrolyte being maintained at a temperature ofbetween about 5 and about 45 F.

7. In a process for the production of an oxide coating onan aluminumsurface, the step comprising subjecting said surface to electrolyticoxidation in an electrolyte consisting essentially of an aqueoussolution of mellitic acid and sulfuric acid, the current density beinggradually raised from about 6.0 amperes per square foot to at least 36amperes per square foot and maintained thereat for a period of at least20 minutes.

8. In a process for the production of. an oxide coating on an aluminumsurface, the step comprising subjecting said surface to electrolyticoxidation in an electrolyte consisting essentially of an aqueoussolution ofmellitic acid and chromic acid, the current density beinggradually raised fromabout 3.0 amperes per square foot to at least 40amperes per square foot and maintained thereat for a pcriodof atleast 20minutes.

9. Ina process for the production of an oxide coating on an aluminumsurface, the step comprising subjecting said surface to electrolyticoxidation in an electrolyte consisting essentially of an aqueoussolution of mellitic acid and phosphoric acid, the current density beinggradually raised from about 10 amperes per square foot to at least 40amperes per square foot and maintained thereat for a period of at least20 minutes.

10. In a process for the production of an oxide coating on an aluminumsurface, the step comprising subjecting said surface to electrolyticoxidation in an electrolyte consisting essentially of an aqueoussolution of mellitic acid and oxalic acid, the current density beinggradually raised from about 10 amperes per square foot to at least 40amperes per square foot and maintained thereat for a period of at least20 minutes.

11. In an electrolytic oxidation process for the pro'duction of an oxidecoating on an aluminum surface, the steps comprising, immersing saidsurface in an electrolyte consisting essentially of an aqueous solutionof mellitic acid and sulfuric acid, maintaining the temperature of saidelectrolyte between about and about 45 F., vigorously agitating saidelectrolyte, initially applying a current having a density of about 6.0amperes per square foot, gradually raising said current density to about36 amperes per square foot, and maintaining said current density thereatfor a period of at least 20 minutes.

12. In an electrolytic oxidation process for the production of an oxidecoating on an aluminum surface, the steps comprising, immersing saidsurface in an electrolyte consisting essentially of an aqueous solutionof between about 0.5 and about 1.5% by weight of mellitic acid andbetween about 7.0 and about 13% by weight of sulfuric acid, maintainingthe temperature of said electrolyte between about 5 and about 45 F.,vigorously agitating said electrolyte, initially applying a currenthaving a density of about 6.0 amperes per square foot, gradually raisingsaid current density to about 36 amperes per square foot, andmaintaining said current density thereat for a period of at least 20minutes.

13. In an electrolytic oxidation process for the production of an oxidecoating on an aluminum surface, the steps comprising, immersing saidsurface in an electrolyte consisting essentially of an aqueous solutionof between about 0.5 and about 1.5% by weight of mellitic acid andbetween about 7.0 and about by weight of chromic acid, maintaining thetemperature of said electrolyte between about 5 and about 45 F.,vigorously agitating said electrolyte, initially applying a currenthaving a density of about 3.0 amperes per square foot, gradually raisingsaid current density to about 40 amperes per square foot, andmaintaining said current density thereat for a period of at least 20minutes.

14. In an electrolytic oxidation process for the production of an oxidecoating on an aluminum surface, the steps comprising, immersing saidsurface in an electrolyte consisting essentially of an aqueous solutionof between about 0.5 and about 1.5 by weight of mellitic acid andbetween about 10 and about 40% by weight of phosphoric acid, maintainingthe temperature of said electrolyte between about 5 and about 45 F.,vigorously agitating said electrolyte, initially applying a currenthaving a density of about 10 amperes per square foot, gradually raisingsaid current density to about 40 amperes per square foot, andmaintaining said current density thereat for a period of at least 20minutes.

15. In an electrolytic oxidation process for the production of an oxidecoating on an aluminum surface, the steps comprising, immersing saidsurface in an electrolyte consisting essentially of an aqueous solutionof between about 0.5 and about 1.5 by weight of mellitic acid andbetween about 5.0 and about 10% by weight of oxalic acid, maintainingthe temperature of said electrolyte between about 5 and about 45 F.,vigorously agitating said electrolyte, initially applying a currenthaving a density of about 10 amperes per square foot, gradually raisingsaid current density to about 40 amperes per square foot, andmaintaining said current density thereat for a period of at least 20minutes.

References Cited in the file of this patent UNITED STATES PATENTS2,111,377 Wales Mar. 15, 1938 2,692,851 Burrows Oct. 26, 1954 2,743,221Sanford Apr. 24, 1956 FOREIGN PATENTS 447,421 Great Britain May 18, 1936OTHER REFERENCES Heilbron: Dictionary of Organic Compounds, vol. 2(1936), pages 545 and 546,

1. IN A PROCESS FOR THE PRODUCTION OF AN OXIDE COATING ON AN ALUMINUMSURFACE, THE STEP COMPRISING SUBJECTING SAID SURFACE TO ELECTROLYTICOXIDATION IN AN ELECTROLYTE CONSISTING ESSENTIALLY OF AN AQUEOUSSOLUTION OF MELLITIC ACID AND AN ACID SELECTED FROM THE GROUP CONSISTINGOF SULFURIC ACID, CHROMIC ACID, PHOSPHORIC ACID, OXALIC ACID ANDMIXTURES THEREOF.